You can plant tomatoes outside when all danger of frost has passed, and the outdoor temperatures meet these conditions:

• Nighttime temperatures should consistently stay above 50°F (10°C), ideally above 55°F (13°C) for fruit setting.
• Daytime temperatures should be consistently between 65 to 70°F (18 to 21°C).
• Soil temperature should have warmed to at least 60°F (16°C), ideally 65 to 70°F (18 to 21°C).

For most areas, this means planting tomatoes outdoors in late spring, typically after the last frost date in your region. It's important not to plant too early, as cold soil and frost can kill tomato plants or stunt their growth. If you start seeds indoors, transplant the tomato plants outside when the seedlings are about 15-20 cm tall with at least four true leaves and the temperature conditions are met. You should also harden off the seedlings before planting them outside. In cooler zones, waiting a bit later to avoid any late frosts or cold snaps is advisable, and protective measures like raised beds or coverings can help with cold spells. Warmer zones may allow planting earlier and even a second crop later in the season. A quick practical test for soil warmth is to thrust your finger into the soil; if it's uncomfortable to keep your finger in the soil for a full minute, the soil is likely too cold for planting tomatoes. To sum up, plant tomatoes outside after the last frost when night temperatures are reliably above 50°F (10°C) and soil temperatures are at least 60°F (16°C) for best results.